Question

could you tell E.coli from Enterobacteria aerogenes using carbohydrates fermentation tubes? which of the IMViC tests would let you tell these two species apart?

Answer 1

Escherichia coli and Enterobacter aerogens are members of family Enterobactericeae.

E.coli and Enterobacter aerogens are gram negative, non sporing, non acid bacillus. E.coli and Enterobacter aerogens exhibit general morphological characters and are difficult to differentiate on the basis of morphology. Biochemical tests are done to differentiate E.coli and Enterobacter aerogens.

Escherichia coli and Enterobacter aerogens are both carbohydrate fermenting organisms and can ferment glucose, lactose, mannitol and sucrose. Both the organism ferment these sugars to produce acid and gas.

Note: Typical strain of E.coli cannot​​​​​ ferment Sucrose.

IMViC test are used primarily for the identification of Escherichia coli and Enterobacter aerogenes. The IMViC series of tests include Indole production, Methyl Red test, Voges Proskauer test and Citrate Utilisation.

Indole test aid to differentiate E.coli and Enterobacter aerogenes.

E.coli is Indole positive while Enterobacter aerogenes is negative. E.coli have the ability to degrade the aminoacid tryptophan to indole. Indole can be easily detected with Kovac's reagent. Kovac's reagent is composed of p- ​​dimethylaminobenzaldehyde, butanol and HCl. Indole is extracted from the medium ( tryptone broth) into the reagent layer by the acidified alcohol component and forms a cherry red complex with the p​​​​dimethylaminobenzaldehyde.

Enterobacter aerogenes is Indole negative.